The invention relates to security systems and particularly to intrusion detectors that utilize a passive infrared sensor (PIR) to detect the presence of an intruder within the protected area by sensing the heat emitted by the intruder's body. The PIR sensor has a detector that outputs a signal that is proportional to the difference between the intruder's body temperature and the background temperature of the sensor's field of view. This signal is amplified to levels compatible with the sensor's processing logic. The processing logic contains a thresholding means and will output an alarm signal if a minimum threshold is exceeded. One of the major drawbacks with this arrangement is that an alarm signal is generated when the minimum threshold is exceeded regardless of the source of the signal For example, an active space heater in the detector's field of view will generate a signal approximately ten times larger than that of an intruder. Unfortunately, the processing circuit is blind to the upper bound of the signal and generates a false alarm signal.